1. Field of the Invention
The present invention relates to a polishing apparatus that polishes an inner circumferential surface of a disk-shaped substrate inner circumference, for example, a glass substrate for a magnetic recording medium or the like, a polishing brush and a manufacturing method of the disk-shaped substrate.
2. Description of the Related Art
In recent years, the production of disk substrates as disk-shaped substrates has been activated, under increased demands as recording media. As a magnetic disk substrate as one of the disk substrates, an aluminum substrate and a glass substrate are used widely. The aluminum substrate is characterized by its high processability and low cost, meanwhile the glass substrate is characterized by its excellent strength, surface smoothness, and flatness. In particular, requirements for compact size and high density of disk substrates recently have become extremely high, and the glass substrate of which surface roughness is small and that enables high density has attracted a lot of attention.
As the related art with regard to such a manufacturing equipment that manufactures a magnetic disk substrate described in official gazettes, there is an art of polishing the inner circumferential surface of a glass disk including a portion having a hole at the center (for example, refer to patent documents 1 and 2).
In the patent document 1, piled glass disks are set rotatably around the central axis, and a shaft mounted polishing brush having numerous brush bristles around its axis is inserted into the portions having a hole at the center of the piled glass disks. Thereafter, this shaft mounted polishing brush is reciprocated, while the shaft is rotated in the reverse direction to the rotation direction of the piled glass disk, and thereby the inner circumferential surfaces of the piled glass disks are polished.
Further, in the patent document 2, a polishing method is proposed where glass substrates are soaked in polishing liquid including separate abrasive, and thereby insufficient polishing and polishing failure due to liquid shortage are prevented. Furthermore, in the patent document 2, an art is disclosed where brush bristles implanted spirally on rotation axis are rotated to polish the surfaces, and fresh polishing liquid is always circulated and supplied to the surfaces to be polished, and thereby polishing efficiency, reproducibility and precision are increased.
FIG. 17 shows an example of a conventional method of polishing an inner circumferential surface of a disk-shaped substrate according to the related art.
In the polishing method shown in FIG. 17, piled workpieces 501 configured by piling a large number of disk-shaped substrates 10 in the center-axis direction are mounted on a rotating table (not shown in the figure), with the axial direction set vertically. Into a portion having a hole at the center 501A of the piled workpieces 501, a polishing brush 502 of which shape is like a shaft connected to a rotating driving shaft that is not shown in the figure at the upper end is inserted and arranged. In addition, on the top of the piled workpieces 501, nozzles 503 that supply polishing liquid including abrasive are arranged.
While the polishing liquid is supplied from the nozzles 503 to the upper face of the piled workpieces 501, the piled workpieces 501 are rotated and the polishing brush 502 is also rotated and reciprocated in the axial direction so that an inner circumferential surface of the portion having the hole at the center 501A of the piled workpieces 501 (that is, the inner circumferential surface of the disk-shaped substrate 10) is polished.
On the polishing brush 502, a brush rows is provided spirally with a predetermined pitch having a predetermined interval (a portion having brush clearance) around a shaft core. By this arrangement, the portion having the brush clearance is also formed spirally with the same pitch as that of the brush rows.
In the polishing method as shown in FIG. 17, the polishing liquid supplied to the upper face of the piled workpieces 501 flows as shown by arrows in FIG. 17 and is supplied to a polishing work area (between the polishing brush 502 and the inner circumferential surface of the piled workpieces 501) during the polishing work. That is, the polishing liquid flows into the portion having the hole at the center 501A of the piled workpieces 501, passes through the portion having the brush clearance of the polishing brush 502 and reaches the polishing work surface. Then, the polishing liquid is moved downward by the spiral action of the portion having the brush clearance caused by rotation of the polishing brush 502 and finally drained downward from the lower end of the portion having the hole at the center 501A of the piled workpieces 501.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 11-33886.
[Patent document 2]
Japanese Unexamined Patent Application Publication No. 11-221742.
Here, in the above conventional polishing method of polishing the inner circumferential surface, polishing efficiency is varied depending on rigidity (brush hardness) of the polishing brush as a whole.
That is, by using a polishing brush with higher brush hardness, rapid polishing may be realized. As a result, working efficiency is improved, and productivity is raised, which contributes to cost reduction. On the other hand, if the brush hardness of the polishing brush is too high, polishing is uneven and finishing accuracy is lowered.
Therefore, in order to carry out stably and efficiently polishing work with high precision, a polishing brush with appropriate brush hardness and brush bristle materials collecting at the center thereof needs to be used.
However, the brush hardness of the polishing brush is varied not only by various factors such as materials, wire diameter, length and the like of the brush bristle material but also by bundling form of the brush bristle material. Thus, for example, even if a brush of which brush bristle material, wire diameter and length are controlled is used, there may be a case where stable polishing is not performed.
Further, in the above conventional polishing method of polishing the inner circumferential surface, it is difficult to supply polishing liquid evenly and sufficiently in the vertical direction of the piled workpieces which is a polishing work area, therefore, polishing becomes uneven in the vertical direction of the piled workpieces.
That is, as mentioned above, during the polishing work, the polishing liquid supplied to the upper face of the piled workpieces passes through a narrow portion having the brush clearance from the upper face and reaches the polishing work area and moves gradually downward while polishing. Therefore, the amount of the polishing liquid supplied to the polishing work area is limited, and since the location where the polishing liquid is supplied is limited to the upper face, the amount of the polishing liquid used in the polishing is largely varied depending on the location in the piled workpieces. As a result, the polished state of the inner circumference polishing differs depending on the location in the piled workpieces, and it becomes difficult to perform inner circumference polishing with little unevenness and high precision with respect to individual disk substrates configuring the piled workpieces.
Accordingly, the present invention is made in consideration of the above-mentioned problems. An object of an illustrative, non-limiting embodiment of the present invention is to overcome the disadvantage described above and to provide a polishing apparatus that promotes supply of the polishing liquid to the polishing work area and performs even polishing in the piled direction of the piled workpieces, and the like.
Another object is to provide a polishing brush for carrying out stably and efficiently the polishing work with a high precision.